1. Field of the Invention
This invention relates to a curable silicone release composition and a release sheet having a cured coating of the composition formed thereon.
2. Prior Art
For the purpose of preventing the adhesion or bond between substrates such as paper sheets and plastic films and tacky materials, it has been a common practice to form cured coatings of silicone composition on the substrate surface to impart release properties thereto. Known methods for forming silicone coatings on the substrate surface include (i) a method of effecting addition reaction between an organopolysiloxane having an aliphatic unsaturated group and an organohydrogenpolysiloxane in the presence of a platinum compound catalyst to form a releasable coating and (ii) a method of effecting condensation of an organopolysiloxane in the presence of an organic acid metal salt catalyst such as organic tin compounds to form a releasable coating.
With respect to release properties, such silicone coatings are generally required to ensure light release at low speeds. Silicone coatings allowing for light release at high speeds are also needed in order to accommodate high speed rewinding and high speed label debris removal. A demand for the solventless type is now increasing from the standpoints of safety and hygiene.
Of the above-mentioned methods, the method that can produce cured silicone coatings featuring improved cure and light release at high speeds is the method (i) of effecting addition reaction between an organopolysiloxane having an aliphatic unsaturated group and an organohydrogenpolysiloxane. Included in the organopolysiloxane having an aliphatic unsaturated group are chain siloxanes of the following general formula, for example. ##STR1## Such chain siloxanes, however, are unsuccessful in providing light release at either low or high speeds.
In JP-A 251465/1988 and JP-B 19267/1991, compositions comprising an aliphatic unsaturated group-bearing organopolysiloxane containing RSiO.sub.3/2 units are disclosed as achieving light release at high speeds and improving curing properties.
The aliphatic unsaturated group-bearing organopolysiloxane used in these compositions contains two or more RSiO.sub.3/2 units. Such compounds containing a plurality of RSiO.sub.3/2 units in a molecule become predominant of three-dimensional structure and have the problems of difficult control of their molecule, a widely varying viscosity at the end of preparation, and a short pot life. From the view point of flow, these compounds are no longer Newtonian fluids. It is then difficult to apply such a compound in a solventless system to form a thin coating although the coating must be as thin as 1 .mu.m or less in order to be effective for the release treatment of plastic films and paper sheets.